1. Field of the Invention
The present invention relates to a technique for generating a model for a planar multiple layer film stack structure and in particular to modifying this model for a binary grating.
2. Related Art
Examples of multiple layer binary grating structures are found on integrated circuit devices during various stages of their production. For example, arrays of periodic structures used to form various parts of those devices typically form one or more binary multiple layer grating structures. FIG. 1 illustrates a cross-section of an exemplary binary multiple layer grating structure (hereinafter grating structure) 100 formed on a silicon (Si) substrate 101. In this case, each film stack of grating structure 100 includes a vertically-inclined, Si member 102, a thermal silicon dioxide (SiO2) film 104, and a silicon nitride (SiN) cap 103. An SiO2 (oxide) layer 105 is formed over and completely covers the film stacks.
Note that grating structure 100 is shown in idealized form with film stacks having straight edges and constant (or constantly varying) thickness. In a real chip, each film stack typically has irregular edges and large local thickness variations (e.g. on the order of 100 Å per mm). Thus, determining an accurate thickness range for binary layers of grating structure 100 remains a difficult problem in the chip fabrication industry.
An accurate thickness determination can be critical in performing certain processes. For example, accurately determining the thickness of a binary layer including SiN caps 103 can become critical when oxide layer 105 is polished, e.g. using chemical-mechanical polishing (CMP). That is, one step in a typical fabrication recipe is to polish oxide layer 105 to a predetermined thickness, which is measured from the top of SiN caps 103. To the extent that the binary layers of grating structure 100 vary in thickness, determining when to stop polishing oxide layer 105 becomes uncertain.
Note that spectroscopic ellipsometry (SE) can measure the changes in the state of polarization of light upon reflection from a surface to determine the thicknesses of multiple continuous films (i.e. thin homogeneous layers), wherein each continuous film is larger than the light spot used to analyze that continuous film. Unfortunately, grating structure 100 comprises multiple “discontinuous” (i.e. non-homogeneous) films, thereby rendering the relatively simple SE models inaccurate for measuring the thickness of grating structure layers.
In another known technique, full diffraction theory can be used to model a grating structure. However, the computational requirements for full diffraction modeling may be too large for some commercial applications.
Therefore, a need arises for a technique to accurately measure and estimate the thickness of various layers of a grating structure while minimizing computational resources.